The VLA-COSMOS Survey: III. Further Catalog Analysis and the Radio Source Counts

TL;DR

This paper analyzes the VLA-COSMOS radio survey data to correct for observational biases, assess catalog completeness, and derive accurate radio source counts down to 60 microJy, revealing an upturn and potential decline in source counts.

Contribution

It provides a detailed correction for bandwidth smearing and noise biases, and presents the most precise radio source counts at 1.4 GHz for flux densities below 1 mJy.

Findings

Bandwidth smearing correction factor of about 0.8-0.9 applied.

Radio source counts confirm the upturn at ~0.5 mJy.

Evidence of a decline in source counts below 0.1 mJy.

Abstract

The VLA-COSMOS Large Project has imaged the 2 sq.deg. COSMOS field with a resolution of 1.5 arcsec and a sensitivity of about 11 microJy (1 sigma) yielding to a catalog of about 3600 radio sources. In this paper we present a further analysis of the VLA-COSMOS Large Project catalog of radio sources aimed to: 1) quantify and correct for the effect of bandwidth smearing in the catalog, 2) determine the incompleteness produced by the noise bias and the resolution bias in the new catalog and 3) derive the radio source counts at 1.4 GHz. The effect of bandwidth smearing on the radio sources in the catalog was quantified comparing the peak and total flux densities in the final mosaic and in each of the individual pointings where the source was closest to the center of the field. We find that the peak flux densities in the original VLA-COSMOS Large Project catalog have to be divided by a factor about 0.8 or 0.9, depending on the distance from the mosaic center. The completeness of the radio catalog has been tested using samples of simulated radio sources with different angular size distributions. These simulated sources have been added to the radio image and recovered using the same techniques used to produce the radio catalog. The fraction of missed sources as a function of the total flux density is a direct measure of the incompleteness. Finally, we derived the radio source counts down to 60 microJy with unprecedented good statistics. Comparison to the findings of other surveys shows good agreement in the flux density range 0.06-1 mJy confirming the upturn at about 0.5 mJy and a possible decline of the source counts below about 0.1 mJy.